Plastic or organic resin substrates are often coated with coating compositions to form a surface protective coating for the purposes of imparting high hardness and mar resistance. Known in the art are coating compositions comprising (partial) hydrolyzates of hydrolyzable organosilanes and coating compositions further comprising colloidal silica.
For example, JP-A S51-002736, JP-A S53-130732, and JP-A S63-168470 disclose coating compositions comprising an organoalkoxysilane, a hydrolyzate of the organoalkoxysilane and/or partial condensate thereof, and colloidal silica, wherein an excess of water is present for converting alkoxy radicals into silanol. In most cases, however, primers must be previously coated onto organic resin substrates in order to help surface protective coatings of these coating compositions firmly adhere to the substrates. The commonly used primers are vinyl polymers in solvent solution. The coatings of these coating compositions have a high hardness and weathering resistance enough to provide good protection for substrates, but are less tough and prone to crack. The coatings lack storage stability in that their hardness decreases with the lapse of time.
Abrasion-resistant silicone coating compositions which can be applied to organic substrates without a need for primers are disclosed in JP-B S63-35675 and JP-A H11-293197. These compositions use polar solvents that can attack the substrates, such as ethyl acetate, 2-butanone and benzyl alcohol.
For the same purpose of substrate attacking, U.S. Pat. No. 4,486,504 and U.S. Pat. No. 5,411,807 propose the addition of (meth)acrylic acid or hydroxyl-containing esters thereof.
Merely adding such substrate-attacking solvents or compounds is effective in improving initial adhesion, but insufficient in achieving long-term or wet adhesion. Another problem of these techniques is that the substrate can be whitened when an excess of the substrate-attacking compound is added for the purpose of improving adhesion.
The addition of an adhesion promoter is another approach for the abrasion-resistant silicone coating compositions which can be applied to organic substrates without a need for primers. The adhesion promoters proposed thus far include binders having phenyltrihydroxysilane incorporated therein (JP-A S60-79071) and (meth)acrylated polyurethane, reactive radical-containing acrylic copolymers, and caprolactone based polyester polyols (JP-A 2001-247769, U.S. Pat. No. 5,503,935, and U.S. Pat. No. 5,349,002). The use of such adhesion promoters are still insufficient in adhesion to resin substrates, especially long-term adhesion.
Although many attempts were made to improve the adhesion of abrasion-resistant silicone coating compositions to organic resin substrates as discussed above, none are fully satisfactory. There is a strong desire to have a coating composition for forming a cured coating which exhibits mar resistance and long-term substrate adhesion while maintaining transparency.